The present invention relates to an explosive detection system, and specifically to an explosive detection system for detecting vapor generated from an explosive or the like sensitively to judge whether or not such an explosive is present in luggage, cargo or the like.
As techniques in the prior art for detecting an explosive, the typical example of which is a nitrocompound, to judge whether or not there is such an explosive, there are known methods disclosed in U.S. Pat. Nos. 4,987,767 and 5,109,691. These methods include the steps of collecting vapor from a person or an object to be inspected, concentrating the gas chemically, heating and desorbing the gas, and examining the gas using a detector, such as an ion mobility analyzer or a gas chromatograph having an electron capture detector.
On the other hand, U.S. Pat. Nos. 4,580,440 and 4,718,268 disclose a mass spectrometer using an atmosphere-chemically ionizing procedure based on corona discharge. This method includes the steps of concentrating a sample gas chemically, heating and desorbing the gas, and examining the gas with an atmospheric pressure chemical ionization quadrupole mass spectrometer.
In the approaches disclosed in the U.S. Pat. Nos. 4,987,767 and 5,109,691, the sensitivity of the used detector is insufficient. Thus, it is necessary for a detection to be made via the step of chemical concentration. However, if the step of chemical concentration is added, the time required for the chemical concentration, which depends on the degree of the concentration, becomes substantial (several minutes several tens of minutes). For this reason, much time is required for a single analysis, resulting in a problem that it is impossible to carry out continuous and speedy analysis of a lot of specimens. Therefore, there arises a problem in that detection of any explosive inside luggage or cargo at an airport, which is required to be quickly carried out, cannot be realized. How to collect a sample gas from luggage or cargo is also brought into question.
In the approaches disclosed in the U.S. Pat. Nos. 4,580,440 and 4,718,268, a high-sensitivity detector is used, such as an atmospheric pressure ionization mass spectrometer using corona discharge. However, a sufficient capability cannot be exhibited by such a device, so that the step of concentration must be used. Thus, it is impossible to examine a lot of specimens continuously and quickly. In the same manner as in the prior art described above, therefore, there remains a problem in that detection of any explosive inside luggage or cargo at an airport, which is required to be quickly carried out, cannot be realized.
The reason why continuous and speedy examination cannot be achieved in the prior art disclosed in the U.S. Pat. Nos. 4,580,440 and 4,718,268 is as follows. As illustrated in FIG. 13 in the U.S. Pat. No. 4,580,440, firstly, a method is employed in which a sample gas is introduced from a concentration device 62 into an introduction region 102. Thus, much time is required for concentration, heating and desorption of the sample so that the sample gas cannot be introduced continuously and quickly from the specimen. Therefore, this method cannot be applied to detection of any explosive inside luggage and cargo at an airport. Secondly, when the sample gas is introduced from the concentration device 62 into the introduction region 62, the concentration of the sample gas itself introduced into the introduction region 102 is lowered because of abrupt diffusion, which occurs because the volume of the introduction region 102 itself is abruptly enlarged. As a result thereof, there remains a problem in that the detection sensitivity is lowered. Thirdly, the sample gas is not heated in the chamber 102 (the method for heating the sample gas introduced into the chamber 102 is not sufficiently disclosed), and there remains a problem in that the ionization efficiency by corona discharge is bad. Fourthly, the direction of introduction of the sample gas to an orifice 110 for introducing ions into an evacuated mass analyzing region is not sufficiently considered, resulting in a problem in that both the ionization efficiency in a corona discharge region and the efficiency of taking in ions from the orifice 110 are bad. After all, the poor ionization efficiency by corona discharge, as described above, results in the following: the ionization efficiency must be raised by adding a reaction gas (poisonous hydrogen chloride, and the like) from a chemical reaction gas source 100. The fact that poisonous hydrogen chloride is used in inspection of luggage at an airport is very undesirable.
Furthermore, for explosive-detection the publications described above disclose only independent use of a gas detection system. The improvement in detection efficiency of an explosive by combining a gas detection system with an X-ray inspection system or the like has not been investigated. Moreover, the connection of the system in the prior art to any X-ray inspection system is actually difficult since continuous and speedy inspections cannot be carried out.
In order to solve the above-mentioned problems, the explosive detection system of the present invention comprises a sample introduction region for introducing a gas sample to be inspected, a corona discharge region for corona-discharging the introduced gas sample negatively, and a mass analyzing region for subjecting the ions generated by the corona discharge region to mass analysis. That is, according to the present invention, the fact that explosives, the typical example of which is a nitrocompound, are liable to be ionized negatively is used, and they are ionized by negative corona discharge. The generated negative ions are measured by a mass spectrometer. Since the negative ionization efficiency exhibited by the negative corona discharge is very high, the detection sensitivity is also sufficiently high. For this reason, any complicated chemical concentration step, as in the above-mentioned prior art, becomes unnecessary. By using, in particular, an ion trap mass spectrometer, which is a mass spectrometer including an ion-storing capability, high-speed physical concentration can be used in the mass analyzing region, instead of chemical concentration. High-sensitivity detection can be made without any loss of performance of high-speed detection.
On the other hand, in order to make continuous and speedy inspections by using an atmospheric pressure chemical ionization mass spectrometer, a pipe or the like which can be heated is used so as to forward vapor from a specimen (that is, a sample gas) continuously to an ionization region in the corona discharge region using a gas forwarding pump. At this time, in order to reduce absorption or the like of the sample gas in the pipe unit, a great deal of gas must be introduced at a high speed into the ionization region.
In accordance with the present invention, a mechanism is provided for preventing the gas temperature of the corona discharge region from being lowered and for heating the sample gas efficiently. The drop in the concentration of the sample gas in the corona discharge region is suppressed by setting the volume of the inside of the ionization region so as not to be significantly larger than that of the pipe and further causing the sample gas to be efficiently introduced into a corona discharge space without diffusion. Furthermore, if the flow of the introduced gas is made substantially parallel to the line connecting an aperture for taking ions in a vacuum and the corona discharge region, the ionization efficiency is raised and simultaneously the efficiency of introducing the generated ions into the aperture is raised.
Detection sensitivity can be highly improved to make continuous and speedy inspections of specimens possible by raising the ionization efficiency in the corona discharge region in the above-mentioned manner. The use of any poisonous reaction reagent becomes unnecessary. As a result, the present invention can be applied to inspection of luggage at an airport, and the like. Simultaneously, the present invention can be used together with an X-ray inspection system, so that the capability of inspection and detection of explosives can be improved.
According to the present invention, nitrocompounds having plural nitro groups can be detected with high sensitivity. These nitrocompounds are susceptible to being negatively ionized, and negative ions are easily produced by negative corona discharge. Therefore, very high sensitivity can be attained. Moreover, any sample can be highly concentrated in this mass spectrometer by using, as a mass spectrometer, an ion trap mass spectrometer, inside of which ions can be stored. Therefore, even when the vapor pressure of an object to be inspected, such as an explosive, is very low, an inspection can be certainly made. However, a quadrupole mass spectrometer and a magnetic sector type mass spectrometer can be used as well.